1. Field of the Invention
The present invention relates generally to imaging systems, and particularly large depth-of-field imaging systems, and more particularly to large depth-of-field iris recognition systems.
2. Technical Background
Humans have a number of unique physical characteristics, such as fingerprints, iris patterns, and voices, whose measurement allows for distinguishing one person from another. The science of measuring such characteristics is known as “biometrics.”
The measurement of a person's iris pattern to perform biometric identification (“authentication”) is called “iris recognition.” Iris recognition involves taking an image of a person's iris and then using pattern recognition techniques to create the equivalent of a “fingerprint” of the eye. Unlike fingerprints, however, iris images are obtained in a less obtrusive manner than fingerprints because they do not require physical contact with the subject.
Early work on iris recognition systems is described in the article by John G. Daugman, “How iris recognition works.” IEEE transactions on Circuits and Systems for Video Technology, 14(1), January 2004, pp. 21-30, in U.S. Pat. No. 5,291,560 to Daugman, entitled “Biometric personal identification system based on iris analysis,” and in U.S. Pat. No. 4,641,349 to Flom et al. entitled “Iris recognition system,” which article and which patents are incorporated by reference herein.
Iris recognition requires that one or both irises be imaged with relatively high resolution so that as much iris detail as possible can be captured. The imaging must also be performed over a relatively large depth of field because the iris is three-dimensional and because some latitude is required in the exact position of the subject's eye relative to the optical system that captures the iris image. The optical system must also have a field-of-view (FOV) sufficient to capture not only the entire iris but also a portion of the face surrounding the eye to perform the necessary image processing.
A major shortcoming of many iris recognition systems is that the optical system requirements of large depth of field, sufficiently large FOV, and high-resolution are somewhat contradictory. For example, a high-resolution imaging system requires a relatively large numerical aperture (NA) (i.e., a low F/#), while a large depth of field requires a relatively low NA (or high F/#). A relatively high NA also decreases the light-gathering ability of the optical system so that longer exposure is required to capture an iris image. This, in turn, requires that the subject remain steady and that their eyes not move (or that they not blink) for a longer period of time so that the iris image is not blurred or obstructed.
A number of different approaches have been used to overcome the challenges posed by the optical design, including using wavefront coding and relatively complex optical systems. However, the need remains for an iris recognition system that has a simple yet robust optical system that provides the requisite optical imaging performance over a wide range of conditions and situations.